Sintered metal oxide articles and methods of making

ABSTRACT

Methods of making metal oxide articles, preferably iron oxide articles, and articles thereby produced. The method comprises the steps of slightly pressing powder to a compact, the powder consisting essentially of a first oxide of the metal; and subjecting the compact to a heat treatment that causes the powder to sinter into a unitary body and results in the transformation of at least a portion of the first oxide to a second oxide by oxidation or deoxidation during the heat treatment. In disclosed embodiments, the heat treatment is conducted either in air at atmospheric pressure or at a subatmospheric pressure. The method optionally includes more heating/cooling steps resulting in additional oxidation/deoxidation cycles. Sintered iron oxide articles of the invention have high mechanical strengths and interconnected pore structures, providing for efficient filtering of liquids and gases.

FIELD OF THE INVENTION

[0001] The present invention relates to methods of making sintered metaloxide articles having desired mechanical properties and interconnectedpore structures, and oxide articles thereby produced.

BACKGROUND

[0002] Sintering of inorganic powder compacts into useful solid productsis a common and efficient way of fabricating metals, ceramics, andcermets. The general pattern of ceramic sintering includes threestages—initial, intermediate, and final. In the initial stage, the poreshape may vary greatly depending on the size and geometry of particlecontacts, and the pore structure is open and fully interconnected. Inthe intermediate stage, where the porosity typically shrinks to 40-10%,the pores become smoother and typically have a cylindrical structurethat remains interconnected. The open pore network becomes unstable whenthe porosity is reduced to about 8%; then, the cylindrical porescollapse into spherical pores, which become closed and isolated. Theappearance of isolated pores manifests the beginning of the final stageof sintering, leading to the densest products.

[0003] Major efforts in ceramic sintering have been made to obtainadvanced materials such as electronic ceramics, structural ceramics, andhigh toughness composites where desired properties are sought to bereached at maximal densification (minimal porosity). The use of ceramicmaterials that have been sintered through only the intermediatesintering stage, however, has been more limited. One such use of thesematerials is in the filtration of gases and liquids. Among ceramic metaloxides, filter materials which have been obtained are commonly made ofalumina (Al₂O₃), zirconia (ZrO₂), and aluminum-silicates.

[0004] The intrinsic properties of iron oxides, hematite (α-Fe₂O₃) andmagnetite (Fe₃O₄), make them well-suited for diverse applications. Theseoxides are among the least expensive and naturally abundant substances.They are refractory ceramic materials that are chemically stable invarious gas and liquid media, hematite being particularly appropriatefor use in corrosive and oxidative environments. Furthermore, hematiteand magnetite are environmentally benign, which make them suitable forwater filtration and various applications in food, wine, pharmaceuticaland other industries where environmental and health requirements areparamount. Moreover, hematite is electrically non-conductive andnon-magnetic, and magnetite is highly conductive and magnetic, so thetwo iron oxides cover a wide spectrum of desirable electric and magneticproperties.

[0005] There exist numerous methods to prepare hematite and magnetitepowders to be used as powders in various applications. However, there isa need in efficient and practical (economical) processes of makingmechanically strong hematite and magnetite articles by sintering therespective powders, particularly into filter materials. U.S. Pat. No.3,984,229 discusses attempts to briquette iron oxide raw materials atelevated temperatures of 800° C. to 1100° C. and concludes that it hasbeen impossible to find a sufficiently strong material for the briquettemolds (col. 1, lines 60-68). U.S. Pat. No. 5,512,195 describes efficienttransformation of hematite powder into a magnetite single phase bymixing hematite powder with various organic substances, serving as abinder and reducing agent, and sintering at 1200° C. to 1450° C. in aninert gas. The strength of the sintered magnetite phase and its porestructure have not been characterized.

[0006] To obtain strong sintered articles, high pressure isconventionally employed. For example, U.S. Pat. No. 4,019,239 describesmanufacturing magnetite articles by sintering and hot compactingmagnetite powder in air at 900° C. to 1300° C. and a pressure of 100 to600 MPa (1000 to 6000 atm), leading to a dense body with a porosity lessthan 3%.

[0007] In addition to high pressure requirements, conventional sinteringof metal oxide powders usually requires binders and other extraneousagents to shape a powder preform and obtain the desirable composition.For example, in U.S. Pat. No. 5,512,195, sintering of hematite powder toa magnetite single phase requires mixing hematite powder with variousorganic substances that serve as binders and reducing agents. Bycontrast, the sintering of hematite powder without incorporation of anyorganic substance at 1200° C. to 1450° C. in an inert gas makes thehematite-magnetite conversion so low that the process is unfit forindustrialization. It would be highly desirable to develop an effectiveand economical sintering process of iron oxides without the use of anyadditives or high pressures.

SUMMARY OF THE INVENTION

[0008] In one aspect, the present invention includes a method of makingmetal oxide articles, and preferably iron oxide articles. The methodcomprises the steps of slightly pressing powder to a compact, the powderconsisting essentially of a first oxide of the metal; and subjecting thecompact to a heat treatment that causes the powder to sinter into aunitary body and results in the transformation of at least a portion ofthe first oxide to a second oxide of the metal. The powder comprises afirst oxide that is substantially free from additives, at least aportion of which is transformed to a second oxide by oxidation ordeoxidation during the method of the present invention. The methodoptionally includes one or more heating/cooling steps during the heattreatment process, resulting in additional oxidation/deoxidation cycles.

[0009] In another aspect, the invention includes sintered metal oxidearticles, and preferably iron oxide articles, made by the method of theinvention.

[0010] One advantage of the present invention is that it providessintered metal oxide articles, and preferably iron oxide articles, ofhigh mechanical strength and other desired mechanical properties.

[0011] Another advantage of the invention is that it provides sinteredmetal oxide articles, and preferably iron oxide articles, havinginterconnected pore structures capable of efficient filtering gases andliquids.

[0012] Yet another advantage of the invention is that it providesefficient and economical processes of making sintered metal oxidearticles, and preferably iron oxide articles, without the need forsintering additives of any kind and/or high pressures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The present invention provides sintered metal oxide articles, andpreferably iron oxide articles, of desired mechanical properties, suchas high strength, and an interconnected pore structure capable ofefficient filtering of gases and liquids. In accordance with theinvention, metal oxide powder is subjected to a heat treatment totransform at least a portion of the oxide into a different oxide. Theheat treatment is conducted at temperatures less than the melting pointsof the oxides and for suitable holding times to sinter the powder into aunitary oxide article. The powders used in the present invention aresaid to consist essentially of metal oxide in that such powders aresubstantially free from other compounds and additives such as binders,reducing agents, and the like.

[0014] Heating regimes for sintering are chosen to cause the oxidationand/or deoxidation of the oxide such that it is transformed to adifferent oxide, with several oxidation/deoxidation cycles possible.Although not wishing to be bound by theory, it is believed that oxygentransport during deoxidation and/or oxidation contributes to effectivesintering and the resulting desired mechanical properties and uniformityin appearance and interconnected pore structure of the sintered articlebody. The invention thus obviates the need for sintering additives andhigh sintering pressures.

[0015] The present invention is described with specific reference toiron oxide articles, and specifically iron oxide filters, that are madeby sintering iron oxide powders. The scope of the invention, however,includes articles of other metal oxide materials, of any form andintended use, that are made by sintering metal oxide powders thatundergo oxidation and/or deoxidation during the sintering process.

[0016] In cited embodiments, sintered iron oxide filters are produced inaccordance with the present invention. For example, in one embodiment,hematite (α-Fe₂O₃) filters are made from magnetite (Fe₃O₄) powder. Inanother embodiment, hematite filters are made from hematite powder,which transforms to magnetite and back to hematite during sintering. Inyet another embodiment, magnetite filters are made from hematite powder.

[0017] The filters are made in a sintering process wherein metal oxidepowder is placed into a mold and hand-pressed into a compact andsubjected to a suitable heat treatment to cause sintering into a unitarybody and oxidative/deoxidative transformation of the powder. Preferredmolds are alumina rings typically having an inner diameter of from about10 to about 70 mm, and a height of from about 3 to about 60 mm. Thepowder particles are of any suitable size for sintering such as, forexample, about 50 to about 200 microns. Such powders are readilyavailable.

[0018] The heat treatment of the invention is selected based on thethermal properties of iron oxides. In air at atmospheric pressure,hematite is stable at elevated temperatures up to about 1350° C. butdecomposes to magnetite at higher temperatures up to about 1450° C.Because magnetite begins to decompose to wustite FeO at temperaturesabove 1450° C., this represents an upper limit of sintering temperaturesin atmospheric air. For subatmospheric pressures, suitable sinteringtemperatures are lower according to the pressure within a vacuumfurnace. In cited embodiments of the present invention, vacuum sinteringtypically occurs at a pressure within the range of about 10⁻⁴ to about10⁻⁵ torr, wherein hematite begins to decompose to magnetite at about750° C. and magnetite begins to melt at about 1300° C. The process isoptimized on the premise that higher sintering temperatures allow forshorter sintering times. At pressures of about 10⁻⁴ to about 10⁻⁵ torr,efficient sintering occurs at 950° C. to 1250° C., preferably at 1000°C. to 1250° C., and more preferably at 1150° C. to 1200° C.

[0019] Embodiments of the present invention are further described withreference to the following non-limiting examples. In all examples, theiron oxide materials, namely hematite and magnetite, are distinguishedby stoichiometry and magnetic properties

EXAMPLE 1 Production of Sintered Hematite Filters from Magnetite Powder

[0020] Hematite filters were made from magnetite powder according to anembodiment of the present invention.

[0021] Magnetite powder was obtained my milling thin-walled magnetitestructures produced in accordance with U.S. Pat. Nos. 5,786,296 and5,814,164, which are incorporated herein by reference. The powder wasseparated on commercial sieves into fractions according to the followingparticle size ranges (in microns): 160 to 100, 100 to 80, 80 to 50 and<50. Portions of each powder fraction were poured into closed-endedmolds to form multiple (e.g., at least three) samples of each powderfraction. The molds were in the form of alumina rings having an internaldiameter of about 11 mm and a height of about 8 mm, placed on a platinumplate serving as the mold bottom. Each sample was compacted by hand witha metal rod to a density of from about 2.3 to about 3.5 g/cm³, andplaced at room temperature into an electrically heated and unsealedfurnace for subsequent heat treatment in atmospheric air.

[0022] Groups of samples were subjected to the following separate heattreatments (all heating rates were about 2° C. per minute), based on theinventors' finding that the sintering process in air was inefficientbelow around 1350° C. but efficient for temperatures up to around 1450°C.:

[0023] (a) Samples of all powder fractions were heated to about 1300° C.and held for about three hours (thus causing a transformation frommagnetite to hematite), then heated to about 1450° C. and held for about15 minutes (thus causing a transformation from hematite to magnetite),and then furnace cooled (thus causing a transformation from magnetite tohematite) (as used herein, “furnace cooled” refers to cooling by leavingsamples in the furnace after sintering and turning off the furnacepower),

[0024] (b) Samples of powder fractions 160 to 100 microns, 100 to 80microns, and 80 to 50 microns were heated to about 1450° C. and held forabout 15 minutes (during which heating, the magnetite transforms tohematite and back to magnetite), then cooled to about 1300° C. and heldfor about three hours (thus causing a transformation from magnetite tohematite), and then furnace cooled.

[0025] (c) Samples of powder fractions 160 to 100 microns were heated toabout 1200° C. and held for about three hours (thus causing atransformation from magnetite to hematite), then heated to about 1450°C. and held for about 15 minutes (thus causing a transformation fromhematite to magnetite), and then furnace cooled (thus causing atransformation from magnetite to hematite).

[0026] As described, each of the heat treatments resulted in more thanone transformation between magnetite and hematite. After the sampleswere cooled to about room temperature, they were removed from the molds.The resulting hematite samples had substantially the same appearance andproperties regardless of variations in the heat treatments employed. Thesample densities were within the range of about 2.3 to about 3.4 g/cm³,which is about 45 to about 65 percent of hematite bulk density (i.e.,corresponding to a porosity of about 55 to about 35 percent,respectively). The samples were mechanically strong enough to be groundby common abrasives, and were characterized by an open interconnectedpore structure capable of effective filtration of liquids. This Examplethus demonstrates a relatively simple method for making strong, uniformiron oxide filters, particularly hematite filters, in accordance withthe present invention.

EXAMPLE 2 Production of Sintered Hematite Filters from Hematite Powder

[0027] Hematite filters were made from hematite powder according to anembodiment of the present invention.

[0028] Hematite powder was obtained my milling thin-walled hematitestructures produced in accordance with U.S. Pat. Nos. 5,786,296 and5,814,164, which are incorporated herein by reference. The powder wasseparated according to size and placed into molds according to Example1.

[0029] Samples (e.g., at least three) were heated at a rate of about 2°C. per minute to about 1450° C., held for about three hours (thuscausing a transformation from hematite to magnetite), and furnace cooled(thus causing a transformation from magnetite to hematite). Theresulting sintered hematite samples could be removed from their moldsbut were mechanically weak such that they were crushed by slight handpressure.

[0030] In an effort to improve mechanical strength, additional powdersamples were heated to about 1300° C. and held for about three hours,then heated to about 1450° C. to about 1500° C., and held for about onehour (thus causing a transformation from hematite to magnetite), andthen furnace cooled (thus causing a transformation from magnetite tohematite). The sintered samples showed only a marginal increase instrength. As such, substantially monotonic heating followed by monotoniccooling was found to be inefficient for producing hematite filters fromhematite powder.

[0031] However, when more cooling/heating steps were added to the heattreatment to provide more oxidation/deoxidation cycles, the resultinghematite samples showed a significant increase in strength. For example,powder samples were heated to about 1250° C. and held for about threehours, then heated to about 1450° C. and held for about 15 minutes (thuscausing a transformation from hematite to magnetite), then cooled toabout 1250° C. and held for about 15 minutes (thus causing atransformation from magnetite to hematite), then heated again to about1450° C. and held for about 15 minutes (thus causing a transformationfrom hematite to magnetite), then cooled again to about 1250° C. andheld for about three hours (thus causing a transformation from magnetiteto hematite), and then furnace cooled. The resulting sintered hematitesamples were strong enough to be ground by common abrasives. Moreover,the resulting samples were uniform in appearance and were characterizedby an open interconnected pore structure.

[0032] Although not wishing to be bound by theory, the significantincrease in filter strength resulting from the severaloxidation/deoxidation cycles may be due to oxygen transport within thesintered body contributing to effective sintering, and to the resultingmechanical properties and uniformity of the sintered article body havingan interconnecting pore structure.

EXAMPLE 3 Production of Sintered Magnetite Filters from Hematite Powder

[0033] Magnetite filters were made from hematite powder according to anembodiment of the present invention.

[0034] Hematite powder was made and separated according to size andplaced into molds according to Example 2. Samples (e.g., at least three)were placed in a vacuum furnace at a pressure of about 10⁻⁴ to about10⁻⁵ torr, heated at a rate of about 8-9° C. per minute to about 1210°C. to about 1250° C. and held for about 5 to about 30 minutes (thuscausing a transformation from hematite to magnetite), and then furnacecooled while maintaining vacuum (thus preventing a transformation frommagnetite to hematite, which would occur in air).

[0035] The sintered magnetite filters were easily removed from theirmolds, and were mechanically strong enough to be ground by commonabrasives. The sample densities were within the range of about 2.3 toabout 3.4 g/cm³, which is about 45 to about 65 percent of magnetite bulkdensity (i.e., corresponding to a porosity of about 55 to about 35percent, respectively), typically increasing with a decrease in theinitial hematite particle size. The sintered samples were uniform andwere characterized by an open interconnected pore structure.

EXAMPLE 4 Production of Sintered Magnetite Filters from Magnetite Powder

[0036] Magnetite filters were made from magnetite powder. Magnetitepower was made, separated according to size and placed into moldsaccording to Example 1. Samples (e.g., at least three) were placed in avacuum furnace at a pressure of about 10⁻⁴ to about 10⁻⁵ torr, heated ata rate of about 8° C. per minute to about 1250° C. and held for about 30minutes, and then furnace cooled.

[0037] In this example, the iron oxide (magnetite) powder did notundergo any transformation to any other iron oxide during sintering. Theresulting sintered magnetite filters were significantly weaker and muchless uniform than the magnetic filters made from hematite powder asdescribed in Example 3. Notably, the weaker magnetite samples in Example4 had, on average, higher densities (up to about 4 g/cm³) than themagnetite samples produced in Example 3. While not wishing to be boundby theory, this unusual inverse relation between strength and densityindicates that in producing samples of high strength theoxidation/deoxidation cycles are more important than simpledensification.

EXAMPLE 5 Evaluation of Sintered Hematite and Magnetite Filters

[0038] The hematite and magnetite filters formed according to Examples 1to 3 were evaluated against standard glass filters with known poresizes. The pore size of each hematite and magnetite filter was estimatedby determining their ability to filter freshly prepared suspensions ofFe(OH)₃, CaCO₃ and Al(OH)₃. Filtration efficiency for all filters wasevaluated by measuring the water filter productivity (“WFP”), which isthe volume of water filtrated per filter unit surface area per unittime, for a given pressure. The results of the filtration testing areshown in Table I.

[0039] The filtration efficiencies for the filters produced inaccordance with Examples 1 to 3 were found to be much greater thanefficiencies for glass filters of comparable pore sizes. For example,for a hematite filter made in accordance with Example 1 from magnetitepowder and having a pore size up to about 40 microns, the WFP was foundto be 829 cm³/cm² min at a pressure of about 10 torr. By comparison, aglass filter having a similar pore size has a WPF of about 100 cm³/cm²min at the same pressure. As another example, for a hematite filter madein accordance with Example 1 from magnetite powder and having a poresize up to about 15 microns, the WFP was found to be 186 cm³/cm² min ata pressure of about 10 torr. By comparison, a glass filter having asimilar pore size has a WPF of about 3 cm³/cm² min at the same pressure.

[0040] Inspection of Table I reveals several structure-propertyrelationships for the sintered filters of the present invention. Forexample, for a given sintering process, a decrease in powder particlesize results in a decrease in filter pore size and an increase in filterdensity. Also, a decrease in powder particle size results in a decreasein WFP. TABLE I Results of filtration testing for sintered iron oxidefilters tested under a pressure of about 10 torr. WFP Fil- SinteredPowder (cm³/ ter Powder Filter fraction Density Pore size cm² no.Material Material (microns) (g/cm³) (microns) min) 1 magnetite hematite160 −   2.4 40 − 15 829 100 2 magnetite hematite 100 − 83 2.7 15 − 10186 3 magnetite hematite  83 − 50 3.1 <10 56 4 hematite magnetite 100 −83 2.5 15 − 10 160 5 hematite magnetite 100 − 83 2.6 40 − 15 159 6hematite hematite 160 −    2.6 100 − 40  179 100 7 hematite hematite 100− 83 2.7 40 − 15 58 8 hematite hematite 100 − 83 2.9 15 − 10 46

[0041] The mechanical strength of filters 1 to 3, as listed in Table I,was evaluated on the basis of crush strength. Crush strength wasmeasured by polishing cylindrical filter samples, having diameters ofabout 10 to about 11 millimeters and heights of about 5 to about 6millimeters, to obtain smooth, parallel top and bottom surfaces. Thesamples were wrapped by a polyethylene film, placed in a press(compressive force about 39 kN), and compressed at a rate of about 0.4atm/sec. The moment of sample crush was distinctly seen on a pressmanometer. These filters were found to have crush strengths of about 30atm, about 200 atm and about 260 atm, respectively, showing a stronginverse correlation with powder particle size. This expected inversecorrelation is an additional indication that the filters of the presentinvention possess a normal interconnected pore structure.

[0042] The results of the filtration demonstrate that the methods of thepresent invention result in the production of strong iron oxide articleshaving an interconnected pore structure suitable for efficientfiltering.

[0043] The present invention provides a novel method of making sinteredmetal oxide articles. The sintered articles of the invention arecharacterized by desired mechanical properties, such as high strength,and an interconnected pore structure. Those with skill in the art mayrecognize various modifications to the embodiments of the inventiondescribed and illustrated herein. Such modifications are meant to becovered by the spirit and scope of the appended claims.

We claim:
 1. A method of making a metal oxide article, comprising thesteps of: pressing powder to a compact, said powder consistingessentially of a first oxide of the metal; and subjecting said compactto a heat treatment, said heat treatment causing said powder to sinterinto a unitary body and resulting in the transformation of at least aportion of said first oxide to a second oxide of the metal.
 2. Themethod of claim 1, wherein said step of subjecting said compact to saidheat treatment comprises the steps of: subjecting said compact to afirst temperature such that at least a portion of said first oxidetransforms to said second oxide; and subjecting said compact to a secondtemperature after said step of subjecting said compact to said firsttemperature, said step of subjecting said compact to said secondtemperature causing at least a portion of said second oxide to transformto said first oxide.
 3. The method of claim 2, wherein said secondtemperature is greater than said first temperature.
 4. The method ofclaim 2, wherein said second temperature is less than said firsttemperature.
 5. The method of claim 2, wherein said heat treatmentfurther comprises the step of subjecting said compact to a thirdtemperature after said step of subjecting said compact to said secondtemperature, thus causing at least a portion of said first oxide totransform to said second oxide.
 6. The method of claim 5, wherein saidthird temperature is greater than said second temperature.
 7. The methodof claim 5, wherein said second temperature is less than said secondtemperature.
 8. The method of claim 1, wherein at least a portion ofsaid heat treatment is conducted at a subatmospheric pressure.
 9. Themethod of claim 1, wherein at least a portion of said heat treatment isconducted in air at atmospheric pressure.
 10. A method of making an ironoxide article, comprising the steps of: pressing powder to a compact,said powder consisting essentially of a first iron oxide; and subjectingsaid compact to a heat treatment, said heat treatment causing saidpowder to sinter into a unitary body and resulting in the transformationof at least a portion of said first iron oxide to a second iron oxide.11. The method of claim 10, wherein said first iron oxide is hematite;said second iron oxide is magnetite; said heat treatment includes thestep of heating said compact to a temperature up to about 1250° C.; andsaid heat treatment is conducted at a subatmospheric pressure.
 12. Themethod of claim 11, wherein said subatmospheric pressure is within therange of about 10⁻⁴ torr to about 10⁻⁵ torr.
 13. The method of claim 10,wherein said step of subjecting said compact to said heat treatmentcomprises the steps of: subjecting said compact to a first temperaturesuch that at least a portion of said first iron oxide transforms to saidsecond iron oxide; subjecting said compact to a second temperature aftersaid step of subjecting said compact to said first temperature, saidstep of subjecting said compact to said second temperature causing atleast a portion of said second iron oxide to transform to said firstiron oxide; and subjecting said compact to a third temperature aftersaid step of subjecting said compact to said second temperature, saidstep of subjecting said compact to said third temperature causing atleast a portion of said first iron oxide to transform to said secondiron oxide.
 14. The method of claim 13, wherein said first iron oxide ismagnetite; said second iron oxide is hematite; said first temperature isup to about 1300° C.; said second temperature is up to about 1450° C.;said third temperature is less than about 1300° C.; and said heattreatment is conducted in air at atmospheric pressure.
 15. The method ofclaim 13, further comprising the step of subjecting said compact to afourth temperature after said step of subjecting said compact to saidthird temperature, said step of subjecting said compact to said fourthtemperature causing at least a portion of said second iron oxide totransform to said first iron oxide.
 16. The method of claim 15, whereinsaid first iron oxide is hematite; said second iron oxide is magnetite;said first temperature is up to about 1450° C.; said second temperatureis up to about 1250° C.; said third temperature is up to about 1450° C.;said fourth temperature is less than about 1300° C.; and said heattreatment is conducted in air at atmospheric pressure.
 17. A sinteredmetal oxide article made by the process of claim
 1. 18. A sintered metaloxide article made by the process of claim
 2. 19. A sintered metal oxidearticle made by the process of claim
 5. 20. A sintered iron oxidearticle made by the process of claim
 10. 21. A sintered iron oxidearticle made by the process of claim
 11. 22. The iron oxide article ofclaim 21, wherein said article has an interconnected pore structurehaving a pore size of up to about 15 microns and a water filterproductivity of at least about 150 cm³/cm² min.
 23. The iron oxidearticle of claim 21, wherein said article has an interconnected porestructure having a pore size of up to about 40 microns and a waterfilter productivity of at least about 150 cm³/cm² min.
 24. The ironoxide article of claim 21, wherein said article has an interconnectedpore structure and a porosity of at least about 35 percent.
 25. Asintered iron oxide article made by the process of claim
 13. 26. Asintered iron oxide article made by the process of claim
 14. 27. Theiron oxide article of claim 26, wherein said article has aninterconnected pore structure having a pore size of up to about 10microns and a water filter productivity of at least about 50 cm³/cm²min.
 28. The iron oxide article of claim 27, wherein said article has acrush strength of at least about 260 atmospheres.
 29. The iron oxidearticle of claim 26, wherein said article has an interconnected porestructure having a pore size of up to about 15 microns and a waterfilter productivity of at least about 175 cm³/cm² min.
 30. The ironoxide article of claim 29, wherein said article has a crush strength ofat least about 200 atmospheres.
 31. The iron oxide article of claim 26,wherein said article has an interconnected pore structure and a porosityof at least about 35 percent.
 32. The iron oxide article of claim 26,wherein said article has an interconnected pore structure having a poresize of up to about 40 microns and a water filter productivity of atleast about 800 cm³/cm² min.
 33. The iron oxide article of claim 32,wherein said article has a crush strength of at least about 30atmospheres.
 34. A sintered iron oxide article made by the process ofclaim
 15. 35. A sintered iron oxide article made by the process of claim16.
 36. The iron oxide article of claim 35, wherein said article has aninterconnected pore structure having a pore size of up to about 15microns and a water filter productivity of at least about 40 cm³/cm²min.
 37. The iron oxide article of claim 35, wherein said article has aninterconnected pore structure having a pore size of up to about 40microns and a water filter productivity of at least about 50 cm³/cm²min.
 38. The iron oxide article of claim 35, wherein said article has aninterconnected pore structure having a pore size of up to about 100microns and a water filter productivity of at least about 175 cm³/cm²min.